Conventionally, as a fluid flow rate measurement device of this type, a flow rate measurement device described with reference to FIG. 9 is generally known (see, for example, Patent Literature 1). FIG. 9 is a block diagram of a conventional flow rate measurement device.
As shown in FIG. 9, a conventional flow rate measurement device includes first oscillator 102 and second oscillator 103 provided at an upstream side and a downstream side of flow path 101, respectively, in such a manner that they face each other. At this time, void arrow 104 in flow path 101 shows a flow direction of a fluid flowing in flow path 101. First oscillator 102 and second oscillator 103 are disposed such that propagation path 105 of an ultrasonic wave transmitted and received by first oscillator 102 and second oscillator 103 intersects at angle θ with the flow direction of the fluid in flow path 101 shown by alternate long and short dashed lines.
Hereinafter, an operation for measuring a flow rate of a fluid by using a conventional flow rate measurement device is described with reference to FIG. 9.
Firstly, as shown in FIG. 9, a transmitted signal is transferred from transmitting unit 107 to first oscillator 102 at the upstream side through switching unit 108. First oscillator 102 is driven by the transmitted signal, and an ultrasonic wave based on the transmitted signal is transmitted into flow path 101.
Then, the ultrasonic wave that propagates through flow path 101 is received by second oscillator 103 provided at the downstream side. A received ultrasonic wave signal is transferred to receiving unit 109 through switching unit 108. At this time, time measurement unit 110 measures time during which the ultrasonic wave is transmitted from first oscillator 102 and received by second oscillator 103.
Next, a transmitted signal is transferred from transmitting unit 107 to second oscillator 103 at the downstream side through switching unit 108. Second oscillator 103 is driven by the transmitted signal, and an ultrasonic wave based on the transmitted signal is transmitted into flow path 101.
Then, the ultrasonic wave that propagates through flow path 101 is received by first oscillator 102 provided at the upstream side. A received ultrasonic wave signal is transferred to receiving unit 109 through switching unit 108. At this time, as mentioned above, time measurement unit 110 measures time during which the ultrasonic wave is transmitted from second oscillator 103 and received by first oscillator 102.
Note here that in order to increase time resolution, a so-called sing-around measurement method is used. In the sing-around measurement method, an operation of transferring the ultrasonic wave signal received by receiving unit 109 to transmitting unit 107 via time measurement unit 110 is repeated, for example, twice to 256 times. At this time, time measurement unit 110 measures a repeated number and total time of the operation of transferring the ultrasonic wave signal from receiving unit 109 to transmitting unit 107.
Hereinafter, a general method for calculating the flow rate and the flow velocity of a fluid to be measured by using a flow rate measurement device is described specifically.
Note here that effective length between first oscillator 102 and second oscillator 103 is denoted by L, a flow velocity of a fluid is denoted by V, a sound velocity of an ultrasonic wave propagating in the fluid is denoted by C, an intersection angle made by a flow direction of the fluid and a propagation direction of the ultrasonic wave is denoted by θ.
At this time, propagation time Ta of the ultrasonic wave from first oscillator 102 at the upstream side to second oscillator 103 at the downstream side and propagation time Tb of the ultrasonic wave from second oscillator 103 at the downstream side to first oscillator 102 at the upstream side are represented as follows.Ta=L/(C+V cos θ)  (1)Tb=L/(C−V cos θ)  (2)Then, mathematical formulae (1) and (2) are deformed, the following mathematical formulae are obtained:C+V cos θ=L/Ta  (3)C−V cos θ=L/Tb  (4)Furthermore, when an addition of formulae (3) and (4) is carried out, the following mathematical formula is obtained:2×C=L(1/Ta+1/Tb)Thus, sound velocity C of the ultrasonic wave can be calculated as shown in the mathematical formula (5):C=(L/2)×(1/Ta+1/Tb)  (5)
Furthermore, flow velocity V of the fluid is obtained as follows. When mathematical formula (4) is subtracted from mathematical formula (3),2×V cos(θ)=L(1/Ta−1/Tb)is obtained. Thus, flow velocity V of the fluid can be obtained from the following mathematical formula (6):V=L/2×cos θ×(1/Ta−1/Tb)  (6)
Herein, effective length L between an ultrasonic wave transmitter/receiver composed of first oscillator 102 and second oscillator 103, and intersection angle θ are previously determined constant numbers.
Therefore, when propagation time Ta and propagation time Tb of the ultrasonic wave are measured by using time measurement unit 110, flow velocity V of the fluid can be obtained from mathematical formula (6).
Furthermore, when a previously determined sectional area of flow path 101 is multiplied, flow rate Q of the fluid can be obtained.
The above-described calculation processing is executed in flow rate calculation unit 111 shown in FIG. 9, flow velocity V of the fluid and flow rate Q of the fluid can be calculated.
Hereinafter, a general measurement method of propagation time of an ultrasonic wave signal in a conventional flow rate measurement device is described by using FIG. 10.
FIG. 10 is a timing chart showing a received wave of a conventional flow rate measurement device.
Note here that FIG. 10 shows signal waveforms transmitted or received by first oscillator 102 and second oscillator 103. Rectangular wave 113 in FIG. 10 shows a transmitted signal applied to first oscillator 102 or second oscillator 103 as a voltage value. Furthermore, received signal 114 having a sinusoidal waveform in FIG. 10 shows received signal 114 received in first oscillator 102 or second oscillator 103 and amplified as a voltage value.
At this time, as shown in FIG. 10, in general, as a receiving point of the time measurement, zero crossing point 116 after received signal 114 having a sinusoidal waveform exceeds a predetermined threshold voltage value shown by broken line 115 is used in many cases.
In other words, a rise time point of rectangular wave 113 of a transmitted signal shown in FIG. 10 is time Tst, that is, transmission start time, and zero crossing point 116 is time Tar, that is, receiving time.
Therefore, propagation time Ta, Tb of received signal 114 that is a measured ultrasonic wave signal is time between time Tar and time Tst, that is, Ta, Tb=Tar−Tst.
However, as is apparent from received signal 114 of FIG. 10, the time at which the propagating ultrasonic wave is received by first oscillator 102 or second oscillator 103 is exactly time Tre that is a lead of received signal 114. In other words, time delay Td between time Tre and time Tar shown in FIG. 10 can be regarded as time delay Td in which the ultrasonic wave reaches first oscillator 102 or second oscillator 103 at a receiving side, and then is received by receiving unit 109.
At this time, time delay Td is largely dependent upon each property of first oscillator 102 or second oscillator 103. Therefore, propagation time Ta of the ultrasonic wave from first oscillator 102 at the upstream side to second oscillator 103 at the downstream side includes time delay (referred to as “Td9”) which is determined by the property of second oscillator 103 at the downstream side, that is, an ultrasonic wave transmitter/receiver at the receiving side. Furthermore, propagation time Tb of the ultrasonic wave from second oscillator 103 at the downstream side to first oscillator 102 at the upstream side includes time delay (referred to as “Td8”) which is determined by the property of first oscillator 102 at the upstream side, that is, an ultrasonic wave transmitter/receiver at the receiving side. FIG. 10 shows that time delay Td9 of second oscillator 103 and time delay Td8 of first oscillator 102 are the same as each other. However, in general, these time delays are different depending upon the property of first oscillator 102 or second oscillator 103.
Thus, in a conventional flow rate measurement device, as shown in FIG. 9, inherent time delay Td8 and time delay Td9 which are determined by the property of ultrasonic wave transmitters/receivers such as first oscillator 102 or second oscillator 103 are stored as offset values in offset value memory section 112. Then, at the time of calculation of the flow rate by flow rate calculation unit 111 as described above, subtraction processing is carried out by subtracting each of offset value Td8 and offset value Td9 from each of propagation time Ta and propagation time Tb of the ultrasonic wave measured in first oscillator 102 or second oscillator 103. Thus, more accurate propagation time (Tpr) of the ultrasonic wave is obtained. As a result, a more accurate flow rate value of a fluid to be measured can be calculated.
Specifically, the offset values at the upstream and downstream sides are defined as time that is 2.5 times as long as the period of the received waveform received by first oscillator 102 and second oscillator 103. Then, the offset value of the ultrasonic wave transmitter/receiver at the receiving side is subtracted from the measured propagation time of the ultrasonic wave. Thus, a flow rate value of the fluid to be measured can be calculated with high accuracy, and a highly accurate flow rate measurement device can be achieved.
However, when an offset value generated from the property of each of first oscillator 102 and second oscillator 103 is previously set, and correction of the flow rate measurement is carried out, the correction cannot be carried out if a temperature change occurs. That is to say, in a conventional flow rate measurement device, when a slight temperature change occurs, a temperature offset according to the temperature change, which includes an electronic circuit at a receiving side, cannot be corrected.